import java.util.*;
class Student{
	private String id;
	private String name;
	public Student(String id,String name){
		this.id = id;
		this.name = name;
	}
	public String toString(){
		return id + ":" +name; 
	}
	public int hashCode(){
		return id.hashCode();
	}
	
	public boolean equals(Object obj){
		if(this==obj){
			return true;
		}
		if(!(obj instanceof Student)){
				return false;
			}
		Student stu = (Student)obj;
		boolean b =this.id.equals(stu.id);
		return b;
		
	}
}

public class Example11{
	public static void main(String[] args){
		HashSet hs = new HashSet();
		Student stu1 = new Student ("1","Jack");
		Student stu2 = new Student ("2","Rose");
		Student stu3 = new Student ("2","Rose");
		hs.add(stu1);
		hs.add(stu2);
		hs.add(stu3);
		System.out.println(hs);
	}
}